The present invention relates to an energy protection method and device for an aircraft, notably a transport airplane.
More specifically, this device is intended to protect the aircraft against situations of low energy (low speed, high incidence, low engine thrust), notably close to the ground.
Generally, such a device comprises, as described for example in the document FR-2 890 645 or in the document FR-2 973 777:                a monitoring unit for monitoring, during a flight of the aircraft, automatically and repetitively, a plurality of data of the aircraft so as to be able to detect a low energy situation, for which conditions relating to a low energy are fulfilled; and an activation unit for automatically activating, in case of the detection of such a low energy situation, a protection function called ALPHA FLOOR, consisting in automatically engaging an autothrust system (“ATHR”) and in automatically controlling the engines of the aircraft for them to supply a maximum thrust.        
The ALPHA FLOOR protection function which is defined in a control and guidance computer of FCGC (“Flight Control and Guidance Computer”) type, therefore generates, in the cases where the aircraft is in a low energy situation, an automatic increase in the thrust to the maximum engine thrust, and does so regardless of the initial position of the throttle controls of the aircraft.
The activation of the ALPHA FLOOR protection function generates, generally, the following actions:                a display of a corresponding message on a flight mode indicator of the aircraft of FMA (“Flight Mode Annunciator”) type;        an application of the maximum engine thrust; and        an automatic engagement of the autothrust (ATHR).        
However, it can sometimes happen that one of the engines of the aircraft cannot engage a protection mode corresponding to the ALPHA FLOOR protection function, for example, in the event of failure. If the implementation of the ALPHA FLOOR protection function was authorized in such a case, it would lead to a strong thrust imbalance (the valid engine applying the maximum thrust, and the failed engine applying the minimum thrust, for an aircraft provided with two engines), which would make the aircraft difficult to control, particularly at low speed. This situation therefore requires particular management.